The field of the present invention is compressors and expanders having high pressure ratios requiring multiple stages.
Where high pressure ratios are desired across a fluid handling apparatus in either expansion or compression, more than a single stage may be required. The arrangement and size of the stages in such equipment are determined by gas dynamics, mechanical limitations and dimensional constraints. Such units may employ a single shaft with multiple wheels thereon with the fluid moving from one wheel to the next. Alternatively, multiple shafts may be employed with wheels mounted to each shaft. In the multi-shaft arrangement, a power transmission device is required such as a gear, coupling or the like. The transmission device transfers the torque by coupling the stages together mechanically where significant losses can occur.
The design of wheels in fluid handling apparatus is based on the actual volume of flow, among other variables. The channel shape varies with the intended fluid volume for optimum performance. In rotary fluid handling apparatus technology, the measure of such channel shape variations is reflected in a nondimensional number called specific speed. A wheel with low specific speed will have a narrow, more radial flow channel. A wheel with high specific speed will have a wide channel and a more axial flow. Low and high specific speed wheels have lower efficiency performance than medium specific speed wheels. Specific speed is defined as follows: EQU N.sub.s =(1/H.sup.3/4) RPM (ACV).sup.1/2
where
RPM rotation speed PA1 ACV actual cubic volume PA1 H turbomachine head
Due to changes in the process fluid in pressure or temperature or both, fluid density may not remain constant. Depending on the compression or expansion duty, the fluid actual volume decreases or increases accordingly. This presents a deviation from the theoretical fluid actual volume for which the wheel was designed, resulting in decreased efficiency.